Aging results in a progressive decline in immune function, which leads to increased morbidity and mortality related to infections. In men, increasing age also results in significant perturbations in the levels of circulating androgens (testosterone and dehydroepiandrosterone (DHEA)), which has been linked to sarcopenia, osteoporosis, cardiovascular disease and diabetes. Since sex steroid levels modulate immune function, it is likely that the age-related decline in androgen levels will also affect immune senescence. Although ~70% of men over the age of 80 meet the clinical definition of hypogonadism, there is controversy over treating this population. Nevertheless, the number of testosterone prescriptions has increased dramatically in the US reaching 5.6 million in 2011. Thus, it is critical that we gain a better understanding of the pleiotropic effects of androgen supplementation in aged men. Dr. Urbanski (co-PI) has developed a novel paradigm of androgen replacement in which testosterone and DHEA are administered to mimic the natural circadian rhythm of these hormones. This approach can restore both testosterone and DHEA levels to youthful 24-hour rhythms in aged male rhesus macaques. We propose to use this highly innovative paradigm to test the hypothesis that physiological testosterone supplementation, designed to mimic the circulating 24- hour pattern of young animals, will ameliorate major biomarkers of immune senescence and improve T cell responses to vaccination. We will first determine the impact of physiological androgen supplementation on the severity of immune senescence by assessing: 1) the frequency of naive and memory T and B cell subsets; 2) thymic output; 3) plasma levels of key inflammatory cytokines; and inflammatory cytokine production by 4) T cells and 5) innate immune cells. We will then characterize the impact of physiological androgen supplementation on the anti-viral immune response following vaccination with modified vaccinia ankara as well as the seasonal influenza vaccine by measuring: 1) kinetics and magnitude of T and B cell proliferation; 2) frequency of responding T cells; and 3) IgG antibody titer. Finally, we will examine the impact of androgen supplementation on the homeostasis of tissue resident lymphocytes by determining 1) T and B cell subset distribution in bone marrow, lymph nodes, bronchoalveolar lavage, and gut biopsies; and 2) the cytokine milieu of these tissues. These studies will be carried out in close collaboration with Dr. Urbanski's laboratory and leverage a funded NIH R01 study (R01 AG-036670) that is investigating the impact of androgen supplementation on cognitive performance and circadian sleep-wake cycles in aged male macaques. By studying the same animals, we will be able to carry out secondary analyses to uncover additional correlations between immunity and neuro- endocrine function in the aged primate. These efforts have the potential to provide a new therapeutic approach to improve immunity in the growing elderly population.